Inkjet types of image-forming apparatuses (inkjet recording apparatuses) are known which form an image by ejecting an ink through a printing head onto a recording medium. Generally, the inkjet image-forming apparatus is capable of forming a highly fine image by employing a small printing head having ink-ejection nozzles in a high nozzle density. The inkjet image-forming apparatus is capable of forming a color image on a recording medium by employing a plurality of the small printing heads and using different color inks for the printing heads with a less expensive smaller constitution. Owing to the above advantages, the inkjet image-forming apparatuses are widely used as various image output apparatuses such as printers, facsimiles, and copying machines for business uses and home uses.
In the above inkjet image-forming apparatuses, it is important to keep the ink in the printing head at a prescribed negative pressure (to keep the pressure exerted on the ink in the printing head to be negative) for stabilizing the ejection operation of the ink through the printing head. For this purpose, generally, a negative pressure-generating means is installed in the ink-feeding system for feeding the ink to the printing head, and the ink kept at the negative pressure by the negative pressure-generating means is fed to the printing head.
A known negative pressure-generating means generates a negative pressure by utilizing capillary action of an ink-absorbing sponge enclosed in the ink tank (e.g., Japanese Patent Application Laid-Open No. 2002-1988). Another known negative pressure-generating means has an energizing means like a spring for energizing outward a flexible member constituting at least a part of an ink tank (e.g., Japanese Patent Application Laid-Open No. 06-155759). A still another known negative pressure-generating means has an ink tank placed below a printing head to apply a negative pressure by utilizing the water head difference (e.g., Japanese Patent Application Laid-Open No. 2003-1844).
The ink kept at a negative pressure by a negative pressure-generating means is fed by suction from the ink tank to a printing head by pressure difference from the negative pressure caused by ink ejection through the printing head. Thereby, the inside of the printing head is kept at a negative pressure constantly.